The present invention relates to a method and apparatus for providing a superelastic metal alloy having improved fatigue life. In particular, the present invention relates to a long fatigue life nickel-titanium alloy wire, ribbon, tubing, or sheet.
There has been great interest in shape memory and superelastic alloys such as nickel-titanium. This family of alloys, also known as nitinol (i.e., Nickel-Titanium Naval Ordinance Laboratory) is typically made from a nearly equal composition of nickel and titanium. Key to exploiting the performance of nitinol alloys is the phase transformation in the crystalline structure that transitions between an austenitic phase and a martensitic phase. The austenitic phase is commonly referred to as the high temperature phase, and the martensitic phase is commonly referred to as the low temperature phase. The back and forth phase changes is the mechanism for achieving superelasticity and the shape memory effect.
As the name implies, shape memory means that the alloy can be twisted into a particular shape in the martensitic phase, and when heated to the austenitic phase, the metal returns to its remembered shape. In contrast, superelasticity refers to the ultra high elastic behavior of the alloy under stress. Typical reversible strains of up to 8 percent elongation can be achieved in a superelastic nitinol wire as compared to 0.5 percent reversible strain in a steel wire, for example. This superelasticity appears in the austenitic phase when stress is applied to the alloy and the alloy changes from the austenitic phase to the martensitic phase. This particular martensitic phase is more precisely described as stress-induced martensite (SIM), which is unstable at temperatures above Af (the austenitic finish) temperature. As such, if the applied stress is removed, the stress-induced martensite reverts back to the austenitic phase. It is understood that this phase change is what enables the characteristic recoverable strains achievable in superelastic nitinol.
Nitinol was originally developed by the military, but has found its way into many commercial applications. Applications that utilize the shape memory effect of the alloy include pipe couplings, orthodontic wires, bone staples, etc. Products that exploit the superelasticity of nitinol include, for example, antennas and eye glass frames.
The medical device industry has also found many uses for nitinol. Nitinol has been used to fabricate guide wires, cardiac pacing leads, prosthetic implants such as stents, intraluminal filters, and tools deployed through a cannula, to name a few. Such devices are taught in, for example, U.S. Pat. Nos. 4,665,906; 5,067,957; 5,190,546; 5,597,378; 6,306,141; and 6,533,805 to Jervis; U.S. Pat. Nos. 5,486,183; 5,509,923; 5,632,746; 5,720,754; 5,749,879; 5,820,628; 5,904,690; 6,004,330; and 6,447,523 to Middleman et al. An embolic filter can be made using nitinol as shown in, for example, U.S. Pat. No. 6,179,859 to Bates et al. Also, implantable stents have been made from nitinol as shown in, for example, U.S. Pat. No. 6,059,810 to Brown; U.S. Pat. No. 6,086,610 to Duerig. A guide wire can be made from nitinol, such as that shown in U.S. Pat. No. 5,341,818 to Abrams. Nitinol is also suitable for the construction of a cardiac harness for treating congestive heart failure as seen in, for example, U.S. Pat. No. 6,595,912 to Lau.
It is understood that all nitinol alloys exhibit both superelasticity and the shape memory effect. To maximize the benefits of each, the industry has developed processing techniques to control these characteristics. Those processing techniques include changing the composition of nickel and titanium, alloying the nickel-titanium with other elements, heat treating the alloy, and mechanical processing of the alloy. For instance, U.S. Pat. No. 4,310,354 to Fountain discloses processes for producing a shape memory nitinol alloy having a desired transition temperature. U.S. Pat. No. 6,106,642 to DiCarlo discloses a process for improving ductility of nitinol. U.S. Pat. No. 5,843,244 to Pelton discloses cold working and annealing a nitinol alloy to lower the Af temperature. United States Publication No. US 2003/0120181A1, published Jun. 26, 2003, is directed to work-hardened pseudoelastic guide wires. U.S. Pat. No. 4,881,981 to Thoma et al. is directed to a process for adjusting the physical and mechanical properties of a shape memory alloy member by increasing the internal stress level of the alloy by cold work and heat treatment.
One characteristic of nitinol that has not been greatly addressed is the cyclic fatigue life. In many devices, especially in medical applications, that undergo cyclic forces, fatigue life is an important consideration. There have been papers delivered on this topic such as W. Harrison, Z. Lin, “The Study of Nitinol Bending Fatigue,” pp. 391-396; M. Reinoehi, et al., “The Influence of Melt Practice on Final Fatigue Properties of Superelastic NiTi Wires,” pp. 397-403; C. Kugler, et at., “Non-Zero Mean Fatigue Test Protocol for NiTi,” pp. 409-417; D. Tolomeo, et at., “Cyclic Properties of Superelastic Nitinol: Design Implications,” pp. 461-471, all published by SMST-2000 Conference Proceedings, The International Organization Of Shape Memory And Superelastic Technology (2001). There is, however, still a need for developing a nitinol alloy that has improved fatigue life especially suitable for medical device applications.